In a geophysical survey, a seismic source can be carried by a truck and positioned at a predetermined location in an area of exploration. The seismic source can be a single axis vibratory source and can impart compressing P-waves into the earth once coupled to the earth and operated. A vibrator 10 according to the prior art is illustrated in FIG. 1A and is diagrammatically illustrated in FIG. 1B. The vibrator 10 transmits force into the ground using a baseplate 20 and a reaction mass 50.
As is typical, the vibrator 10 is mounted on a carrier vehicle (not shown) that uses a mechanism and bars 12/14 to lower the vibrator 10 to the ground. With the vibrator 10 lowered, the weight of the vehicle holds the baseplate 20 engaged with the ground so seismic source signals can be transmitted into the earth. The reaction mass 50 positions directly above baseplate 20 and stilts 52 extend from the baseplate 20 and through the mass 50 to stabilize it.
Internally, the reaction mass 50 has a cylinder 56 formed therein. A vertically extending piston 60 extends through this cylinder 56, and a head 62 on the piston 60 divides the cylinder 56 into upper and lower chambers. The piston 60 connects at its lower end to a hub in a lower cross-piece 54L and extends upward through the cylinder 56. The piston 60's upper end connects to a hub on an upper cross-piece 54U, and the cross pieces 54U-L connect to the stilts 52.
To isolate the baseplate 20 from the bars 14, the bars 14 have feet 16 with isolators 40 disposed between the feet 16 and the baseplate 20. In addition, the feet 16 have tension members 42 interconnected between the edges of the feet 16 and the baseplate 20. The tension members 42 are used to hold the baseplate 20 when the vibrator 10 is raised and lowered to the ground. Finally, shock absorbers 44 are also mounted between the bottom of the feet 16 and the baseplate 20 to isolate vibrations therebetween.
During operation, a controller 80 as shown in FIG. 1B receives signals from a first sensor 85 coupled to the upper cross-piece 54U and receives signals from a second sensor 87 coupled to the reaction mass 50. Based on feedback from these sensors 85/87 and a desired sweep signal for operating the vibrator 10, the controller 80 generates a drive signal to control a servo valve assembly 82. Driven by the drive signal, the servo valve assembly 82 alternatingly routes high pressure hydraulic fluid between a hydraulic fluid supply 84 and upper and lower cylinder piston chambers via ports in the mass 50. As hydraulic fluid alternatingly accumulates in the piston's chambers located immediately above and below the piston head 62, the reaction mass 50 reciprocally vibrates in a vertical direction on the piston 60. In turn, the force generated by the vibrating mass 50 transfers to the baseplate 20 via the stilts 52 and the piston 60 so that the baseplate 20 vibrates at a desired amplitude and frequency or sweep to generate a seismic source signal into the ground.
As the moving reaction mass 50 acts upon the baseplate 20 to impart a seismic source signal into the earth, the signal travels through the earth, reflects at discontinuities and formations, and then travels toward the earth's surface. At the surface, an array of geophone receivers (not shown) coupled to the earth detects the reflected signal, and a recording device records the signals from the geophone receivers. The seismic recorder can use a correlation processor to correlate the computed ground force supplied by the seismic source to the seismic signals received by the geophone receivers.
As can be seen, an essential component of the vibrator 10 is its baseplate 20. FIGS. 2A-2C show the baseplate 20 for the prior art vibrator 10 in plan, side, and end-sectional views. The top of the plate 20 has stilt mounts 24 for the stilts (52; FIG. 1B), and a reinforcement pad 21 surrounds these mounts 24. Retaining ledges 26 are provided for the isolators (40). The long edges near the corners have forked hangers 28 to which ends of the tension members (42) connect, and reinforcement pads 27 are provided around the outside edges of the plate 20 for connecting the shock absorbers (44) to the baseplate 20.
Overall, the baseplate 20 can have a height H1 of about 6.9-in., a width W1 of about 42-in., and a length L1 of about 96-in., and the plate 20 can weight approximately 4020-lbs. As shown in the end section of FIG. 2C, the plate 20 has four internal tubes or beams 30 that run longitudinally along the plate's length. The beams 30 are hollow tubes with rectangular cross-sections and have a height of about 6-in., a width of about 4-in., and a wall thickness of about ⅜-in. Interconnecting spacers 32 position between the beams 30 and between the long cap walls of the baseplate 20.
When operating such a prior art vibrator 10, operators experience problems in accurately imparting desired force into the ground with the vibrator 10 and the baseplate 20. Ideally, operators would like the vibrator 10 to efficiently impart force into the ground with the baseplate 20. Also, operators would like to know the actual ground force applied by the baseplate 20 to the ground when imparting the seismic energy. Unfortunately, the baseplate 20 experiences a great deal of vibration and flexure that can distort or interfere with the ideal operation of the baseplate 20.
Although the typical prior art vibrator and baseplate may be effective, operators are continually seeking more efficient ways to impart seismic energy into the ground for a seismic survey.